Can not override auth config options from tests

When writing auth tests, using the `@helpers.change_config()` decorator on an auth option has no effect. The reason is that auth functions call [`new_authz.py.check_config_permission`](https://github.com/ckan/ckan/blob/04e6ac4c0363bb2923c27ef6bad398b3f73ac819/ckan/new_authz.py#L380) which caches the config and default values the first time is run.

I think is worth not caching the values and allowing them to be overriden from tests than the performance gains.
how about split() with no argument? then it will split on any white-space (and any amount of it) and strips white-space on either side too.